I have vt25 and vt25a also 10y with different brand in my tubes box .i know this tubes have nice sound and balance in all of mid and high and low.
I tray to build tube circuits and test result sound by my self . I receive to vt25 in pre amps circuit. is it possible with this power triode direct filament to build good pre amps?

If you don't increase a lot that coupling capacitor you will have the resonance at approx. 23 Hz with a bad Q factor (poor damping)!!
Very probably you will get a floppy bass.....unless you use very small monitors.

To make it work decently you should go down to 10 Hz with a good damping, at least.
3.3uF looks much better....

This is not my idea and Jim cort has made and test this project . but tanks for your advise.
Jim cort :
I use a higher voltage on the 10Y but it biases just like I wanted... 350V/16mA... The 10Y/801A will take chokes from 150H or up to give a -1dB of a 20Hz.
and:The filament supply is done by my two battery towers which also house 12V batteries for the 26 filament. I can use these to get the 7.5V for the 10Y, no problem, only have to adjust the current regulator resistor to get the right current.so filimant ...

Originally posted by Gluca YES! But You'd better use a transformer to lower the output impedance.

Only true if trying to drive a low impedance input. The output impedance will be around 5k for this operating point. That's fine if the driven amp has an input impedance of at least 50K. Most tube amps are that or higher.

Quote:

Originally posted by 45 This is not a great circuit.

If you don't increase a lot that coupling capacitor you will have the resonance at approx. 23 Hz with a bad Q factor (poor damping)!!
Very probably you will get a floppy bass.....unless you use very small monitors.

To make it work decently you should go down to 10 Hz with a good damping, at least.
3.3uF looks much better....

There's nothing inherently wrong with the circuit. It's a basic parafeed amp. I don't think that damping factor is important here. The amp is driving a low, non reactive load. Just measure the response with the expected load and change the cap value if there is any peaking. Easy enough to do. Here's some related info: http://www.siteswithstyle.com/VoltSe...afeed_fun.html

Originally posted by dave slagle I do not see the parafeed circuit here. It looks like a simple choke loaded cap coupled to a grid resistor. Where is the resonance coming from?

Yes, my bad for misusing the term. Can't really be parafeed in the usual sense if there is no transformer on the output. And without the transformer inductance in parallel with the cap, no resonance either, I guess. In the circuit shown, the inductor has a low Z path to common, but the cap does not, so they are not effectively parallel.

Thanks to for replay on my topic and I think the 10y will work prefect by transformer in matching and lode in out pot stage .but I donít have this transformer in my box .so I should try by capacitor and specially focused on low frequency to find resonance or not !
I have 100 of vt25a they have made for western electric by hitron .these kind of tubes need to supply filament by battery for the best result in clear sound. as I told I have another kind of vt25 from uk by teonex in rebranded .the teonex vt25 is work by ac filament and I found it better in rejection the noise . I checked these two kind of tube in out put circuit in single ended before .

Only true if trying to drive a low impedance input. The output impedance will be around 5k for this operating point. That's fine if the driven amp has an input impedance of at least 50K. Most tube amps are that or higher.

There's nothing inherently wrong with the circuit. It's a basic parafeed amp. I don't think that damping factor is important here. The amp is driving a low, non reactive load. Just measure the response with the expected load and change the cap value if there is any peaking. Easy enough to do. Here's some related info: http://www.siteswithstyle.com/VoltSe...afeed_fun.html

Sheldon

Yes it is for me. Given that load, the frequency response is ok but not the phase and above all the sound!
The inductance and the capacitor are in parallel for the signal.